Bottom dumping railway hopper car

ABSTRACT

A railroad bottom-dumping goldola car has open-topped side and end walls and an open bottom closed by one or more transversely downwardly swinging door or doors, and trucks which run on railroad track rails which are interrupted to form a space over a dumping location below the rails&#39;&#39; level. The car may itself have only one truck at one end, its other end sharing the truck of an adjacent corresponding car of a train of the cars. The car has longitudinally extending rails along its sides on the upper edge of the frame which run on supported horizontal series of rollers supporting the car while it traverses the space between the interrupted rails, and the body&#39;&#39;s swinging door or doors are swung open and shut on hinges, by an operating cam follower wheel journaled to the door in each case and running on a downwardly and upwardly contoured cam rail which spans the space formed by the interrupted rails, the cam rail defining a constant radius equal to the spacing of the hinge axis and the cam follower wheel. This gondola car features a rectangular frame defined by longitudinal sides extending along the car&#39;&#39;s side for its length and transverse ends supported by the trucks, the frame defining an opening providing complete dumping clearance when the door or doors are opened. The frame&#39;&#39;s ends are adapted for coupling to the ends of other frames of corresponding gondola cars to form a train via the common trucks, or in some instances, via the frames themselves, in all instances being made strong enough to carry the forces of jolts and tensions of train operations, preventing these forces from being transmitted to the car&#39;&#39;s walls surrounding the open bottom. A number of the cars may be made up into a train of the articulated type, the adjacent ends of each two adjacent cars sharing the common trucks, thus providing for a greater length of open bottom and its door or doors than if each had its own two trucks.

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1 1 BOTTOM DUMPllNG RAILWAY HOPPER CAR [75] Inventors: Anders Bjorklund; Bert Frelin; Sven Scott, all of Vasteras, Sweden [73] Assignee: Allmanna Svenslra Elektriska Aktiebolaget, Vasteras, Sweden [22] Filed: June 19, 1973 [21] Appl. No.: 371,549

[30] Foreign Application Priority Data June 26, 1972 Sweden 008361/72 [52] US. Cl 214/63; 105/1 R; 105/4 R; 105/165; 105/241 C; 105/250 [51] Int. Cl. B6lld 7/18; B6ld 7/30;B61f 3/12 [58] Field of Search 105/1 R, 3, 4 R, 241 C, 105/253, 1 A, 165, 250; 214/63 [56] References Cited UNITED STATES PATENTS 1,091,466 3/1914 Crawford 214/63 1,889,066 ll/l932 Griffith 214/63 2,708,887 5/1955 Van Alstine 105/4 R X 2,839,010 6/1958 Harbulak 105/1 R 3,220,355 11/1965 Jones 105/253 X 3,352,254 1l/l967 Lauber 105/4 R 3,424,105 H1969 Cripe 105/4 R 3,557,707 1/1971 .loy 105/1 R 3,610,165 lO/l971 Brown et a1.... 105/1 R 3,620,392 11/1971 Greenwood 105/241 C X 3,678,864 7/1972 Gutridge 105/1 R X FORElGN PATENTS OR APPLICATIONS 1,123,870 8/1968 United Kingdom 105/241 C 79,432 7/1932 Sweden 105/3 1,065,557 4/1967 United Kingdom... 105/4 R 1,001,602 10/1951 France 105/241 C 1,408,767 7/1965 France 105/241 C Primary Examiner-Lloyd L. King Assistant ExaminerHoward Beltran Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin A railroad bottom-dumping goldola car has opentopped side and end walls and an open bottom closed by one or more transversely downwardly swinging door or doors, and trucks which run on railroad track rails which are interrupted to form a space over a dumping location below the rails level. The car may itself have only one truck at one end, its other end sharing the truck of an adjacent corresponding car of a train of the cars. The car has longitudinally extending rails along its sides on the upper edge of the frame which run on supported horizontal series of rollers supporting the car while it traverses the space between the interrupted rails, and the bodys swinging door or doors are swung open and shut on hinges, by an operating cam follower wheel journaled to the door in each case and running on a downwardly and upwardly contoured cam rail which spans the space formed by the interrupted rails, the cam rail defining a constant radius equal to the spacing of the hinge axis and the cam follower wheel. This gondola car features a rectangular frame defined by longitudinal sides extending along the cars side for its length and transverse ends supported by the trucks, the frame defining an opening providing complete dumping clearance when the door or doors are opened. The frames ends are adapted for coupling to the ends of other frames of corresponding gondola cars to form a train via the common trucks, or in some instances, via the frames themselves, in all instances being made strong enough to carry the forces of jolts and tensions of train operations, preventing these forces from being transmitted to the cars walls surrounding the open bottom. A number of the cars may be made up into a train of the articulated type, the adjacent ends of each two adja cent cars sharing the common trucks, thus providing for a greater length of open bottom and its door or doors than if each had its own two trucks.

ABSTRACT 1 Claim, 12 Drawing Figures PATENTED JUL 2 9 I975 SHEET Fig. 7

PATENTED JUL2 9 I975 SHEET SHEET PATENTEB JUL29 I975 PATENTED JUL 2 9 I975 SHEET BOTTOM DUMPING RAILWAY HOPPER CAR BACKGROUND OF THE INVENTION A US. railroad dumping gondola car has a body supported at its ends by'railroad trucks of the freight-car type. The bottom is formed by a plurality of hoppers leading to a plurality of doors which are hinged to the body transversely with respect thereto and provided with latches. This permits the body to be transversely braced at a plurality of longitudinally interspaced locations providing the body with good structural strength capable of resisting the jolts and tension forces incidental to its operation when forming a part of a train of cars pulled by a locomotive.

In the above type of car dumping is effected by running the car over tracks on an open-work trestle and releasing the door latches. The discharge of the load, such as coal, metal ore and correspondingly lumpy or granular material, is not as clean and rapid as might be desired. However, this type of car is adapted to be formed into a railroad train of such cars.

A bottom dumping car adapted to run on tracks, such as are used around mines or mills, is disclosed by British Pat. No. 1,123,870, this reference making the full disclosure of that patent a part thereof. Briefly stated, this patent discloses a mine or mill car having substan tially its entire bottom adapted to be opened for cleaner and quicker dumping of its load. The bottom is hinged along side one of the car longitudinally with re spect to the cars side, and swings transversely with re spect to the car. The supporting wheels themselves are fixed to this swinging bottom and while traversing a dumping location the tracks follow curves constituting partial helices, the track contour being such as to swing the door open and closed with a maximum angularity of 45 to the horizontal plane, as the car traverses the dumping location. The upper section of the car body has side rails which ride over a horizontal series of supported rollers which suspend the car body upright as its bottom swings open and closed. In another form of this car the tracks on which the cars supporting wheels are run are interrupted at the dumping location to form a space spanned by a single appropriately contoured cam rail engaged by a single wheeljournaled to the door and which rides this rail to swing the cars bottom open and closed, the single wheel functioning as a cam follower wheel, the cars supporting wheels being freely suspended by the bottom clear from the interrupted rails and forming no part of the dumping action in this instance. i

This car with its transversely swinging bottom has the advantages of providing for opening the entire bottom of the car for rapid and clean dumping while the cars supporting wheels, or single cam follower wheel as the case may be, function to provide a positive acting force causing positive door opening and closing motions while the car traverses the dumping location.

However, because the car is made so that its entire bottom opens and with its supporting wheels fixed to this bottom, the car is unfit to be a part of a train of corresponding cars adapted to run on standard railroad tracks and to be self-propelled by power means. Its bottom cannot provide any structural strength to the up standing walls of its open'topped body and the latter must be made of steel sheets of reasonable gauge, preventing the car from carrying safely the jolts and tensions associated with being part of a propelled train of corresponding cars. If latches are used in an attempt to keep the car bodys side walls and bottom positively closed, while traveling to and from the dumping location, these latches are subjected to severe strains and therefore failures, as the car body flexes and works while carrying the forces incidental to train operation. Finally, articulated passenger car trains have been made up of cars which share common trucks between each two adjacent cars, the trucks being of the passenger car type. This is a railroad passenger car arrangement and it has been restricted to this field.

SUMMARY OF THE INVENTION An object of the present invention is to provide a railroad bottom-dumping gondola car having the advantages of all three types of cars referred to above but free from the disadvantages described.

The invention attains this object by providing a car body of the type having the tranversely swinging door, and possibly a plurality of such doors, each door journaling the cam follower wheel operated by the cam rail while traversing the dumping location as previously described, but with the car bodys upstanding wall structure freed from the jolts and tensions inherent to operation of the car as part of a train of corresponding cars, by being provided with a strong and rigid rectangular frame, defining or encircling or enclosing an open space large enough to provide complete clearance for free passage of the cars load during the bottomdumping operation. This frame, therefore, does not interfere in any way with the dumping operation yet it is strong enough to rigidly resist the jolts and tensions incidental to operation of the car as a component of a train of corresponding cars. The frame is rigid enough to resist deformation and, therefore, provides a substantially non-deflecting part via one of its sides permitting the use of positive-acting latches for holding the swinging door or doors of the ear closed during train operations to and from the dumping location.

Only one end ofthe frame is provided with a truck for running on the railroad rails during normal operation, the other end being constructed to couple with a corresponding truck connected to the end of an adjacent carat that end, the truck end of the frame sharing that truck with the corresponding car adjacent to that end. Thus the car, with other corresponding cars, can form an articulated train adapted to negotiate sharp turns. The rectangular frame of great strength may be provided with the side rails which run over the linear series of supported rollers which carry the car after the train trucks leave the rails interrupted to form the space over the dumping location, the cam follower wheel on each bottom at that time following the contoured cam rail to open and close the door with the latter then swinging open to substantially open the entire bottom of the car body to permit clean and rapid dumping of the cars load.

The truck must be connected to the frame so that the latter can hold the truck suspended during the dumping operation. The truck of each car may be either twowheeled, four-wheeled, or two four-wheeled trucks may be used together, the latter being preferred when the car body is long enough to provide a load weight sufficient to warrant the load carrying ability thus provided. In that case a single bottom door would require an excessively long span over the dumping location necessitating a long length of cam rail and possibly slower 3 dumping action; therefore, with a long car body a plurality of transversely swinging bottom doors is preferred, each provided with its own cam follower wheel, permitting the cam rail to be shorter and of greater curvature or pitch.

Because the car's swinging door or bottom does not itself carry the running wheels of the car, and because it is supported by the rigid rectangular frame, the door may be swung open to a greater angularity than is possible in the case of the prior art car using the transversely swinging door or bottom to which the running wheels are attached. For example, with the present invention maximum door opening angularities may range from 59 to 61 from the horizontal level established by the rectangular frame, an angularity of 60 being preferred. Because the rectangular frame provides complete clearance for dumping, such angularities provide extremely rapid and clean dumping.

Keeping in mind the great weight of the load carried by such a car, such as coal, iron ore or the like, it is considered preferable to provide each car, or alternate ones of the cars in a trail of cars, for example, with its own motive power, such as an electric motor geared to its truck wheels. In this way a train of such cars may operate on a railroad track endless loop with the cars forming an endless train running from loading to dumping locations. Correspondingly, each car may be provided with braking equipment and each rigid rectangular frame may have latching equipment for keeping the doors positively closed but which is remotely controlled to be released while traversing the dumping location, while otherwise positively holding the door or doors closed for each car, keeping in mind that any latch mechanism used is free from the train operating stresses and strains which are carried rigidly by the rectangular frame. Also, the train may be provided with an operating car or station manned by an operator for controlling the individual motorized trucks and brakes of the train.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention are illustrated by the accompanying drawings in which:

FIG. 1 is a side elevation of cars embodying the invention, the cars being relatively short and fourwheeled trucks being involved;

FIG. la shows the intercoupled portions of two cars sharing a common truck, and represents a portion of FIG. 1;

' FIG. 2 is a plane view shcematically showing an articulated train of the cars negotiating a curve;

FIG. 3 is like FIG. 1 but shows an example of fourwheeled trucks used in tandem at each car end, the car being longer and having two bottom dumping doors;

FIG. 4 shows in elevation and partially in vertical section, a dumping location being traversed by one cars of a train of such cars as they are illustrated by FIG. 3;

FIG. 5 is a cross section of FIG. 4, the car being shown in elevation on the left side of the broken line with its bottom door closed, and on the right-hand side of this line. in cross section with the door fully opened;

FIG. 6 is similar to FIG. 1 but shows an example of relatively short cars having a single door having a multiplicity of cam follower wheels for controlling its swinging, and with the car forming part of a train in which two-wheeled running gear is involved;

FIG. 7 is an end wheel of a car showing the manner in which it may be variably banked, depending on the cars running conditions;

FIG. 8 in schematic form, in cross section. shows the dumping operation of a car such as is illustrated by FIG. 6;

FIG. 9 is a plan view schematically showing an articulated train of the car with the trucks running wheels provided with powering means for self-propulsion;

FIG. 10 is a side elevation showing an example of how a train of the new cars, when of the self-powered type, may be provided with a controlling car manned by an operator who controls the train via what may possibly be a normal train operating control system, for example, to control the driving. braking and possibly other operations of the train. this operating control car either leading the train, as in the manner of a locomotive, or being interposed between the cars of the train; and

FIG. 11, in perspective, schematically suggests an example of a suitable rectangular frame, by itself.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Having reference to FIGS. 1 and la of the above drawings, a portion of a railroad train is shown, comprising three railroad bottom dumping railroad gondola cars embodying the principles of the present invention. the cars II and lla being only partially shown while the car 12 is shown completely.

The cars 11 and 12 have their ends coupled together via a common four-wheeled railroad truck 13, and the adjacent ends of the cars Ila and 12 are coupled together through the medium of a corresponding fourwheeled railroad truck 14. The trucks l3 and 14, and the other trucks of the train, all have their wheel axles 13a and 14a journaled in the windows of the trucks side frames via the usual springs (not shown) so that the cars ride easily on the rails shown at R. The springs may work through conventional bolsters (not shown) carrying the weight of the car bodies in the usual manner, but connected to the car bodies so that the trucks can be suspended by the latter while traversing the dumping location.

By making the cars so that they can form an articulated train, with each car sharing the truck of each other adjacent car, a much greater free length of the bottoms of the cars are left free and clear for the dumping when their swinging bottoms, previously described, are swung open by the operation of their cam follower wheels, shown at 15 in FIG. 1, than would be the case if each car had the usual two trucks of the standard bottom-dumping gondola car. By using the common trucks most or all of the bottom of the car body is free for opening. The ends of the car bodies may have declining walls 16 and 16a at both ends leading to their bottom openings, facilitating rapid dumping and freedom from retention of undumped load portions such as might remain if corners were involved.

As hereinafter explained in detail, the cam follower wheel 15 is actuated by the downwardly contoured cam rail while the car is traversing the dumping location, as previously described. the car body at that time being suspended by thepreviously described suspension system, because its wheels 13a and are over the space between the interrupted railroad rails.

FIG. 3 shows an embodiment of the invention, using between each car, two of the-trucks in tandem, as shown at 33a and Ma, joined by longitudinally extending bolsters I7 and 17a respectively, which connect with the adjacent ends of the cars 11!), 12a, I It. The additional load carrying capacity is thus provided because the bodies of the cars are in this instance are made very substantially longer than in the FIG. l and la embodiment. Here again, the maximum possible bottom area is provided for each car. The car length is so long as to make a single door impractical. Therefore. in FIG. 3 two bottom opening doors I8 and 19 are shown, each having its own cam rail follower wheel and 21, respectively, the two doors l8 and 19 being capable of swinging independently with respect to each other.

These doors 1% and 19 swing transversely with respect to the car as previously described, being hinged on axes extending longitudinally with respect to the car. Each door 18 and 19 may have substantially the same dimensions as the door 18a shown in FIG. 1 and which is provided with the single cam follower wheel 15, the latter and the cam follower wheels 20 and 21 all substantially corresponding. Therefore, either length of car may be handled by the same equipment at the dumping location. Here again, the trucks 13a and 14a are shared by each two cars and must be connected to the cars so that the trucks will not fall away from the cars during the times the latter are suspended while traversing the dumping location. Also, the tandem trucks 13a and 140 are in each instance shared by two cars.

Another advantageof using the two doors I8 and 19 for the longer car is that the cam rail at the dumping location can have a contour that is more sharply pitched than it could be if the longer car had a single long door that had to be swung all at once in its entirety.

FIG. 2 schematically illustrates how a number of the cars 22 and 23, interconnected by common trucks 24, can negotiate a curve in the same manner as an articulated passenger train. However, and of substantial importance, there is'the additional advantage that with the cars sharing the trucks the car bottom openings can extend for the maximum possible lengths longitudinally with respect to the cars. i

FIGS. 4 and 5 illustrate the dumping operation. The details are described hereinafter, but it is to be noted that FIG. 5, on the right-hand side of the broken line, serves to show how the entire bottom of the car body must be open to permit the dumping when the bottomdumping doors are opened at the dumping location. Without the features of the present invention the car body would be too weak to permit its incorporation with a number of corresponding cars to form a train that could negotiate railroad tracks without risk of the cars bottom-closing doors inadvertently opening due to the twisting strains that would destroy the efficiency of any latches that would be practical for holding the doors closed.

To overcome this inherent weakness of the prior art cars using a corresponding dumping action, each of the cars is built with its bottom formed by a strong rectan gular frame, shown at F in FIGS. 1 and 2 and at F in FIGS. 3 through 5. In each case the frame has a rectangular contour and is formed by ends which extend transversely for the width of the car and by mutually extending sides which extend for the cars length. This frame may be fabricated from heavy-gauged plate steel or possibly may be a cast steel frame, but in any event it must be strong enough to free the car body portions comprising the upstanding walls, and also the door hinging and latching elements, from twisting motions due to the jolts and tensions due to normal strain opera tions. When built integrally as part of the car body the frame itself may carry the hinge or hinges H, the strong rectangular frame carrying all stresses and strains in a substantially rigid manner. Because of its rectangular shape, the frame internally defines a space co-extensive with the open bottom of the car body. No supporting cross bars, struts or the like are or need be used. it being the strength of the rectangular frame alone that rigidly carries all'of the stresses. The design of such a rectangular frame is well within the skill of structural ,engineers once they have been informed about the present invention.

FIG. 6 serves to show how the principles of the inven tion may be incorporated when using only two wheels between each two adjacent cars. This is done by the use ofa truck 25 journaling the axle of the two wheels and which itself has a forwardly extending portion, described in detail hereinafter, defining the rectangular frame required to practice the present invention.

To reduce or eliminate the normal tension transmitted from car to car in the case of any railroad train, as illustrated by FIG. 9 schematically, each of the cars shown by FIG. 6 may be provided with an electric motor 26 connected to the wheels via gearing 27. This principal may, of course, be applied to all embodiments of the invention, In addition, although not shown, the usual braking and possibly other equipment may be included. FIG. 6 also serves to show how the single door involved by each of the cars shown by this figure, can be provided with two of the cam follower wheels 15a instead of only one 15 as shown in FIG. I, for example. Here, again is a principle that is applicable to the other embodiments. When using two follower wheels for each door, the contour of the cam rail must, of course, provide a linear portion receiving both cam follower wheels at once as required for full door swinging, the trailing one of the wheels opening the door and the leading one closing the door.

Returning to FIG. 6, the rectangular frame forming portion is shown at 28. As illustrated rather schematically by FIG. 11, this frame has its sides and ends defining the large space S required for the dumping operation. The frame is shown by broken lines in FIG. Ill because substantially the same contour and construction is involved when it is a built-in part of the lower portions of the car bodys side and end walls as has been previously illustrated and described. The frame has at one end a king pin hole 29 which cooperates with theking pin 29a of the next cars frame. A stop 30 is provided for holding the cars apart when necessary and to facilitate coupling of the cars together. Preferably power cylinders 31 are provided which are either pneumatically or hydraulically actuated devices, which connect with the wheel axle to facilitate steering when the train goes around curves, as well as power cylinders 32 which are pneumatically or hydraulically actuated devices which operate between the car bodies and the wheel axle of trucks to cause the cars to bank when going around curves, should this be necessary. Each car body may be spaced from the next by means of a stop 33 working against the block 30 of the next car.

FIG. 7 shows how the cylinders 31 may actuate a vertical member 31a fixed to the end of a car body to bank the car through the angle range indicated by broken lines. and FIG. 8 shows how the frame 28 may mount the upstanding walls 34 of the car body. and the transverse swinging doors or door 35 via the side hinges 36, indicated at H in the preceding figures. while providing side rails SR required to cooperate with the linear horizontal series of supporting rollers at the dumping location.

As can be seen in FIG. 8, and more clearly in FIGS. 1 and 5, the side rails SR are positioned along the upper edges of the frames sides. The side rails extend longitudinally from these sides and laterally project for running on the rollers 40a, and while doing so during the dumping operation, the frame is suspended by its top edges via its side rails, as contrasted to extending upwardly from the rollers. Because the frame is suspended from its top edges, greater stability results during the dumping operation.

As previously indicated. a train made up of cars embodying the present invention may have a control or operating unit manned by an operator. This is illustrated by FIG. where the car 37 contains control equipment (not shown) for the various motors 26, indicated in FIG. 9; the braking and other equipment may be controlled in a manner familiar in the field of railroad rolling stock engineering. However, in addition, it is to be noted that the control or operating car 37 is mounted via a frame 28 corresponding to that shown by FIGS. 6 through 8 and 11, so that this car 37 may be positioned at any place within the train. broken lines at the front of the car 37 indicating this fact. In the case of this car 37 the rectangular frame 28 does not function to provide clearance for dumping but instead to permit the car 37 to be placed at any location within a train of cars corresponding to that shown by FIGS. 6 through 8. The same principle can be applied to the other embodiments of the invention.

Returning now to FIGS. 4 and 5, a dumping location is provided by a depression or valley formed below the level of the railroad rails R. Columns 38 and 39 support a beam 40 which extends over a space formed because the railroad rails R are interrupted to define this space, it extending for almost the extent of the beam 40 and the latter supporting the horizontal series of rollers 40a which support the car via its side rails SR while traveling over the space between the interrupted rails, the cars trucks and wheels being then unsupported excepting by their attachment to the car body.

It is to be understood that any of the cars embodying the invention and illustrated by the various figures, may traverse this dumping location. As the loaded car travels along, the cam rail 41 is engaged by the cam follower wheel shown at 42 in FIGS. 4 and 5. The door is swung open in the right-hand side of FIG. 5; the door is swung closed in the left-hand side of FIG. 5. The cam rail 41 .forms two helices and its descending and rising portions at all times are equidistant from the hinge H of the car about a radius equalling the distance extending from the hinge H to the cam follower wheel 41.

While any of the cars in the train are traveling on the railroad rails. the doors may be latched shut, having been shut initially by the action of4l and 42. by means of remote-controlled latching mechanisms which are released manually or automatically as each car approaches the dumping location with the cam follower wheel just engaging the cam rail. such arrangements not being illustrated. However, it is to be noted that due to the rigid rectangular frame incorporated by the cars in each instance. such latching mechanism is relieved from the stresses and strains that would be inherent in the absence of the strong and rigid frame as shown clearly by FIGS. 4 and 5, the open space within the rectangular frame providing complete clearance for dumping.

The angularity of the cam rail 41 indicated in FIG. 4 may be increased or made sharper relative to the plane of the railroad rails by using a multiplicity of doors for the open car bottom when the cars body length is relatively long as is illustrated by FIG. 3.

The dumped material is shown falling into a hopper 43 and through a belt conveyor BC being carried on to its destination.

As indicated in FIG. 11, any of the rectangular frames may be controlled by members 44 cooperating with the next car and controlling balancing valves (not shown) for controlling the actuation of the cylinders 31 to effect axle steering when a train of the cars is rounding a curve.

What is claimed is:

l. A railroad bottom-dumping gondola car comprising a rectangular frame, a gondola body supported by said frame and having a bottom closed by a downwardly swinging door, and railroad wheels normally supporting said frame on railroad tracks, said frame having sides from which longitudinally extending side rails laterally project for running on rollers supporting said frame with said wheels free from railroad tracks during downward swinging of said door for dumping operation; wherein the improvement comprises said side rails being positioned along the upper edges of the frames said sides so that the frame is suspended by the side rails while they run on said rollers. 

1. A railroad bottom-dumping gondola car comprising a rectangular frame, a gondola body supported by said frame and having a bottom closed by a downwardly swinging door, and railroad wheels normally supporting said frame on railroad tracks, said frame having sides from which longitudinally extending side rails laterally project for running on rollers supporting said frame with said wheels free from railroad tracks during downward swinging of said door for dumping operation; wherein the improvement comprises said side rails being positioned along the upper edges of the frame''s said sides so that the frame is suspended by the side rails while they run on said rollers. 